The present invention relates to a multi-hetero-structure waveguide type optical circuit in which various functional circuits are formed upon a wafer by coupling thin-film elements through directional couplers to a waveguide.
In the description of the present invention, the relationship between the prior art active and passive optical elements and a waveguide will be first described. For this purpose, the relationship between a semiconductor laser and a waveguide will be explained.
In the conventional semiconductor laser, a reflector is attached to each end of a laser medium so that the semiconductor laser may function as a resonator. It has been very difficult to attach a waveguide to the resonator for deriving part of the laser light through the reflector. It has been proposed to use a directional coupler to couple a waveguide to a ring resonator type semiconductor laser. However, the attachment of the waveguide is still difficult because part of the laser light is derived. Furthermore, they cannot be fabricated in the form of an integrated circuit. It has been also proposed to use a distribution reflector which is attached to a portion of a thin-film semiconductor laser and formed integral with a waveguide. However, the transmission loss in the waveguide is very large at the wavelength of the laser light because the thin-film waveguide has the same composition as that of the semiconductor laser medium. There is also a drawback in that the output light diverges because the thickness of the thin-film laser medium is made very small in order to reduce the oscillation threshold current. Furthermore, in order to integrate the device with other optical elements, individually fabricated optical elements are required. Moreover, the coupling loss is induced when the optical elements are coupled, and the operation life is short because the crystals are grown over the distribution reflector after the latter has been finished.